Grain Shape Effects on Fluid-Sediment-Structure Interaction Under Oscillatory Flow

The goal of this investigation is to accurately quantify the effects of grain shape and angularity on fluid-sediment-structure interaction under the presence of oscillating fluid motion. Solid objects were subjected to oscillating fluid forcing in a full-scale boundary layer apparatus consisting of a rippled sediment tray oscillating in a still fluid. The variable group included calcareous sediments exhibiting coral, shells and other marine fragments with a variety of shapes and angular features, while the control group consisted of homogenous silica sands with generally smooth and spherical shapes. For the test runs presented here, hydrodynamics were scaled with the object’s Shields parameter, which corresponds to the shear stress acting on the object normalized by its immerse weight, resulting in significantly less mobility and burial on the variable group for streamlined cases. For non-streamlined cases, uniform and symmetric scour patterns were found for the control group as a result of the object pivoting in place. Unstable cases (non-streamlined orientations with object located at ripple crests or flanks) resulted in directional dependencies, with the variable group showing more resistance to coherent motions dominating the near-boundary hydrodynamics. These findings suggest that the irregular nature of the variable group sediments not only increases intergranular friction, but also friction forces between the sediment and the object, thereby hindering its mobilization and burial.